Method for Manufacturing Tube and Fin Heat Exchanger with Reduced Tube Diameter and Optimized Fin Produced Thereby

ABSTRACT

An improved method for manufacturing tube and fin heat exchangers that, according to a preferred embodiment, includes a process for increasing the stiffness and rigidity of heat exchanger fins. Stiffer fins have a greater tendency to maintain proper alignment within a stack of fins, which aids in lacing long stacks of fins with small (e.g., 5 mm) diameter tubing. Preferably, fin stiffness is increased by forming a plurality of longitudinal ribs within the fin during the fin stamping process. More preferably still, two ribs for each longitudinal row of collared holes are provided. The preferred embodiment also includes a slotted heat exchanger fin that is dimensioned and arranged for optimized thermodynamic performance when used with small diameter tubing, thus reducing the space required for a given heat exchanger system.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon provisional application 61/061,498 filedon Jun. 13, 2008, the priority of which is claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to tube and fin heat exchangers, and inparticular, to a novel fin design for tube and fin heat exchangers.

2. Description of the Prior Art

As illustrated in FIG. 1, a typical tube and fin heat exchanger (10)consists of a stack of generally planar metallic fins (12) sandwichedbetween a top end plate (14) and a bottom end plate (16). The terms“top” and “bottom” used for designating heat exchanger end plates arederived based on the heat exchanger orientation during expansion in avertical hairpin expander press and are not necessarily indicative ofthe heat exchanger orientation in any particular installation.

The fins (12) have a number of collared holes (18) formed therethrough,and the top and bottom end plates (14, 16) have corresponding holes (20)formed therethrough. When the fins (12) and end plates (14, 16) arestacked, the holes (18, 20) are in axial alignment for receiving anumber of U-shaped hairpin tubes (“hairpins”) (22) through the stack.Hairpins (22) are formed by bending lengths of small tubes, typicallycopper, aluminum, steel or titanium, 180 degrees around a small diametermandrel. The hairpin tubes (22) are fed, or laced, through theloosely-stacked assembly of fins from the bottom end plate (16) so thatthe open ends (26) of the hairpin tubes (22) extend beyond the top endplate (14). The top end plate (14) is slipped over the open ends (26) ofthe hairpins (22), and the hairpins (22) are mechanically expanded fromwithin to create a tight fit with the fins (12). Finally, return bendfittings (24) are soldered or brazed to the open ends (26) of thehairpin tubes (22) to create a serpentine fluid circuit through thestack of fins (12).

It is advantageous to use hairpin tubing of very small diameter in orderto maximize heat transfer area within a given heat exchanger size andgeometry. Smaller tubes increase the overall heat transfer area and heattransfer coefficient at the refrigerant side of the heat exchanger,which significantly enhances system efficiency. In addition, smallertubing diameter reduces the air flow wake effect behind the heat tube,which reduces the pressure loss due to the presence of the tube facingthe incoming air. Lower pressure loss at the air side reduces the fanmotor power requirement and increases the fin area to further improvethe system heat transfer efficiency. Additionally, the larger the tubediameter, the thicker the tube wall thickness must be in order towithstand a given pressure differential. Therefore, smaller tubediameters allow thinner tube walls for a given refrigerant pressure,which reduces material costs.

According to the present state of the art, the heating, ventilation, andair conditioning (“HVAC”) industry typically manufactures tube and finheat exchangers using hairpin tubes with diameters ranging between 7.0mm and 9.5 mm (⅜ inch). Although the industry desires to manufactureheat exchanger coils of smaller diameter, manufacturing techniques ofprior art have restricted such coils to short lengths, with the resultthat small diameter coils have had limited commercial success. Thesource of the problem is that when the hairpin tubing becomes too small,the lacing process becomes exceedingly difficult, prohibitingcommercially viable manufacturing of any but the shortest heatexchangers. For example, heat exchangers six or more feet in length arereadily manufactured using ⅜ inch copper tubing. However, when 5 mmcopper tubing is used, it has not been commercially feasible to lace aheat exchanger longer than about 36 inches because of the “Chinesehandcuff” effect of the large number of fins. It is desirable,therefore, to provide a manufacturing process that produces a stifferheat exchanger fin produced to ease the lacing process of smalldiameter, e.g., 5 mm or smaller, coils.

The prior art tube-fin exchanger, characterized by 7 mm to ⅜ inchtubing, generally employ fins with a fin width between 19 mm and 22 mm,and a transverse tube pitch ranging between 19 mm and 25.4 mm. Fins ofthese prior art fin dimensions do not deliver optimized performance forsmaller diameter, e.g., 5 mm, tubes. It is also desirable, therefore, toprovide a heat exchanger fin that has enhanced thermodynamic performanceoptimized for small diameter tubing, which results in heat exchangersystems that occupies less space.

3. Identification of the Objects of the Invention

A primary object of the invention is to provide a manufacturing processfor producing stiffer fins to promote the lacing of tube and fin heatexchangers of large size with 5 mm or smaller tubing.

Another object of the invention is to provide a heat exchangermanufacturing process in which heat exchanger fins having a plurality oflongitudinal ribs are utilized to enhance the lacing process.

Another object of the invention is to provide a heat exchanger fin thatis designed and arranged for use with 5 mm or smaller tubing to maximizethermodynamic heat transfer.

Another object of the invention is to provide a heat exchanger fin thatpromotes condensation flow from the fin.

SUMMARY OF THE INVENTION

The objects above as well as other features of the invention arerealized in an improved method for manufacturing tube and fin heatexchangers that, according to a preferred embodiment, includes a processfor increasing the stiffness and rigidity of heat exchanger fins.Stiffer fins have a greater tendency to maintain proper alignment withina stack of fins, which aids in lacing long stacks of fins with small(e.g., 5 mm) diameter tubing. Preferably, fin stiffness is increased byforming a plurality of longitudinal ribs within the fin during the finstamping process. More preferably still, two ribs for each longitudinalrow of collared holes are provided.

The preferred embodiment of the invention also includes a slotted heatexchanger fin that is dimensioned and arranged for optimizedthermodynamic performance when used with small diameter tubing, thusreducing the space required for a given heat exchanger system.

The fin preferably includes slits with ends having a 30 degree incidentangle with respect to the airflow, which helps to re-direct the airflowfrom the tube passing through the collared hole to avoid the wake regionbehind the tube and provides for a more effective air mixture inparallel slits. The angled slit ends also create turbulence at the areaof the fin that has largest distance to neighboring tubes, whichenhances the heat transfer over that area.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail hereinafter on the basis of theembodiments represented in the accompanying figures, in which:

FIG. 1 is a perspective exploded diagram of a typical tube and fin heatexchanger of prior art;

FIG. 2 is a perspective view of a portion of a heat exchanger finarranged for a single longitudinal row of 5 mm hairpin tubes accordingto a first embodiment of the invention, showing a preferred slotpattern, which is repeated between pairs of collared holes, and a pairof longitudinal ribs formed in the fin, which bounds the collared holes;

FIG. 3 is a top view of the portion of the single hairpin row heatexchanger fin of FIG. 2;

FIG. 4 is a perspective view of a portion of a heat exchanger finarranged for two longitudinal rows of 5 mm hairpin tubes according to asecond embodiment of the invention, showing a preferred slot pattern,which is repeated between pairs of collared holes, and two pairs oflongitudinal ribs formed in the fin, which bounds the two longitudinalrows of collared holes;

FIG. 5 is a top view of the portion of the heat exchanger fin of FIG. 4;

FIG. 6 is a bottom view of the portion of the heat exchanger fin of FIG.4;

FIG. 7 is an enlarged cross section view of the heat exchanger fin ofFIG. 4 taken along lines 7-7 of FIG. 5, shown with the collared holes inhidden line to reveal the detail of the raised slots;

FIG. 8 is a left side view (with the front of the fin defined by theincident air flow) of the portion of the heat exchanger fin of FIG. 4;

FIG. 9 is an enlarged cross section view of a longitudinal rib of theportion of heat exchanger fin of FIG. 4 taken along lines 9-9 of FIG. 5;

FIG. 10 is a top view of a portion of the heat exchanger fin of FIG. 4showing the detail and preferred dimensions of pattern of raised slotsfor optimizing thermodynamic performance with 5 mm hairpin tubes; and

FIG. 11 is an enlarged cross section view of a raised vane taken alonglines 11-11 of FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIGS. 2-12 illustrate a fin 12′ dimensioned for small tubing, e.g. 5 mmouter diameter or less, optimized for use with a condenser or evaporatorof a conventional air conditioner. FIGS. 2 and 3 illustrate a heatexchanger fin 12′ according to a first embodiment of the invention thatis characterized by a single longitudinal row of collared holes 18′ foruse in a single-row coil assembly. FIGS. 4-8 illustrate a heat exchangerfin 12′ according to a second embodiment of the invention that containstwo longitudinal rows of collared holes 18′ for use in a double-row coilassembly. However, fins 12′ may be arranged for three, four, five, andsix or more rows of coils according to the invention. The leading andtrailing edges of fin 12′ preferably have corrugated edges.

Referring primarily to FIGS. 2-6, according to the preferred embodimentof the invention, a 5 mm or smaller tube and fin heat exchangermanufacturing process includes a novel and unobvious processing step informing the heat exchanger fins. As with heat exchanger fins 12 of priorart, fins 12′ are formed by a stamping process in a fin press, such asthose produced by Burr Oak Tool, Inc. of Sturgis, Mich. Fin stock isdelivered to a press in a roll of sheet metal. Various metals, heattreatments, and thicknesses may be used, but aluminum is the generalindustry selection. Fin stock is paid out from an uncoiler, lubricated,then fed through a press, where a die draws, details, punches collaredholes, and cuts fins to a desired length and width. Stamping generallyoccurs in several stages.

However, in the preferred manufacturing process, the fin press includesa die that forms two longitudinal ribs 100 into fin 12′ for eachlongitudinal row of collared holes 18′. The purpose of the lengthwisestrengthening ribs 100 is to aid in the fabrication of the coilassembly. Stiffer fins have a greater tendency to maintain properalignment on a lacing table within a stack of fins, which aids in lacinglong stacks of fins with small (e.g., 5 mm) diameter tubing.

Each longitudinal row of collared holes 18′ is disposed between its ownpair of longitudinal ribs 100. For a single row coil arrangement, fin12′ has two ribs 100 (FIGS. 2-3), and for a double-row coil arrangement,fin 12′ has four ribs 100 (FIGS. 4-6). Thus, between adjacent rows oflongitudinal collared holes 18′, there are two longitudinal ribs 100. Ina preferred embodiment of the invention, the height h_(r) (FIG. 9) ofrib 100 above surface 103 of fin 12′ is between 0.05 and 0.25 mm. Morepreferably, h_(r) is about 0.125 mm.

Ribs 100 are also beneficial in the removal of condensate that forms onthe fin during the refrigerant evaporation process. Ribs 100 function toprovide a path for condensate to follow between tubing rows inmultiple-coil arrangements. In single row coil arrangements, ribs 100provide flow paths for condensate on both the leading and trailing edgesof the fin (with respect to the airflow over the fin). Ribs 100 promotethe draining of condensate from the fin 12′, thus minimizing thepotential for condensate carry-over, i.e., condensate blowing off of thefin and becoming entrained in the stream of air flowing across the fins12′.

Heat exchanger capacity and efficiency are determined by both fin areaand tube area. An optimized heat exchanger must properly balance theutilization of fin and tube area to create the best heat transferbetween the refrigerant side and the air side in a cost-effectivemanner. The combination of smaller diameter tubes, e.g., 5 mm orsmaller, with fins 12′ according to the preferred embodiment of theinvention provides optimal heat transfer efficiency andcost-effectiveness.

As best shown by the perspective views of FIGS. 2 and 4, a plurality ofslits 110 are disposed at spaces between the collared holes 18′ within agiven longitudinal row. Each slit 110 forms a projecting or raisedribbon-like segment or vane 112, which is parallel to fin surface 103and is connected at its two longitudinal ends 113 to the surface 103 offin 12′. Segment 112 defines an open portion 114 between the raised vane112 and the fin surface 103 that separates the incoming air flow. Theslit depth dimension d_(v) (along the direction of airflow) (FIGS. 8, 9)is optimized to reduce the boundary layer development on the segment112, which improves heat transfer ability. Preferably, d_(v) rangesbetween 0.5 and 1.5 mm. More preferably, d_(v) equals about 1.0 mm. Theinterval depth d_(i) (FIG. 8) of the fin between adjacent vanes 112 isalso preferably equal to the vane depth d_(v).

Referring to FIG. 10, slits 110 are arranged in an ‘X’-shaped pattern105, with each pattern 105 of slits 110 repeating between each pair ofcollared holes 18′ within a given longitudinal row. In pattern 105,according to the preferred embodiment of the invention, the slits 110are ideally grouped by five longitudinal rows 120, 122, 124, 126, 128,respectively. The leading two rows (on the basis of the direction of airflow) 120, 122, and the trailing two rows 126, 128 each preferablyemploy two slits 110, for which the connecting ends 113 are preferablyformed at an angle α between 15 and 45 degrees with respect to thenormal direction of airflow (airflow being assumed to be perpendicularto the longitudinal direction of the fin). Ideally, α is 30 degrees. Thecenter row 124 preferably employs a single slit 110 with ends 113 formedparallel to the incident airflow. By the nature of the tube and fin heatexchanger, the center portion of fin 12′ that has largest distance toneighboring tubes has the lowest heat transfer efficiency. Pattern 105is designed to guide the airflow to create more turbulence, whichenhances the heat transfer over the area. The angled ends 113 of theslits 110 in first, second, fourth and fifth rows 120, 122, 126, 128create vortices and corresponding turbulence.

Referring to FIG. 5, fin 12′ also provides an optimized and balancedtube distance and fin width for 5 mm tubing. Prior art tube-finexchangers arranged for 7 mm to ⅜ inch diameter tubing have fin widthstypically ranging between 19 mm and 22 mm and transverse tube pitchesranging between 19 mm and 25.4 mm. These prior art fins 12 do notdeliver optimized performance for the smaller tube size, which resultsin a larger space for the heat exchanger system than is necessary usingthe fins 12′ according to the preferred embodiment of the invention. Fin12′, on the other hand, has a reduced fin width dimension p_(w) (i.e.,the distance from center to center between two adjacent collared holes18′ within a single longitudinal row) between 12 and 18 mm and atransverse tube pitch dimension p_(t) (i.e., the perpendicular distancebetween the centerline of two adjacent longitudinal rows of collaredholes 18′) between 10 and 15 mm to give optimized heat transfer capacityand efficiency with minimal use of fin and heat tube material, whichresults in a space efficient product. More preferably, p_(w) is 16 mmand p_(t) is 13.86 mm.

Referring to FIG. 9, the height h_(v) from the top surface of vane 112′to the top surface 103 of fin 12′ preferably ranges from 0.25 to 0.75mm. More preferably still, h_(v) is about 0.5 mm.

The Abstract of the disclosure is written solely for providing theUnited States Patent and Trademark Office and the public at large with away by which to determine quickly from a cursory reading the nature andgist of the technical disclosure, and it represents solely a preferredembodiment and is not indicative of the nature of the invention as awhole.

While some embodiments of the invention have been illustrated in detail,the invention is not limited to the embodiments shown; modifications andadaptations of the above embodiment may occur to those skilled in theart. Such modifications and adaptations are in the spirit and scope ofthe invention as set forth herein:

1. A fin (12′) for a tube and fin heat exchanger (10) comprising: a generally planar metallic sheet; a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′); and first and second longitudinal ribs (100) formed in said sheet, said first longitudinal row of apertures (18′) disposed between said first and second longitudinal ribs (100); said first and second ribs (100) each having a top surface projecting beyond the upper surface (103) defined by said sheet.
 2. The fin (12′) of claim 1 further comprising: a second plurality of apertures (18′) formed through said sheet and defining a second longitudinal row of apertures (18′); and third and fourth longitudinal ribs (100) formed in said sheet, said second longitudinal row of apertures (18′) disposed between said third and fourth longitudinal ribs (100), said second and third longitudinal ribs (100) disposed between said first and second longitudinal lows of apertures (18′).
 3. The fin (12′) of claim 1 further comprising: a plurality of raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′); each of said plurality of raised vanes (112) formed between first and second longitudinal slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet.
 4. The fin (12′) of claim 3 wherein: said plurality of raised vanes (112) are arranged in first, second, third, fourth and fifth rows (120, 122, 124, 126, 128) parallel to said first longitudinal rib (100).
 5. The fin (12′) of claim 3 wherein: at least one of said plurality of raised vanes (112) is attached to the sheet at a first end (113); said first end (113) is oriented at an angle (α) from an imaginary line that is perpendicular to said first longitudinal rib (100); and said angle (α) is between 15 and 45 degrees.
 6. The fin (12′) of claim 5 wherein: said at least one of said plurality of raised vanes (112) is attached to the sheet at a second end (113); said first and second ends (113) are oriented at said angle (α) from an imaginary line that is perpendicular to said first longitudinal rib (100); and said angle (α) is between 25 and 35 degrees.
 7. The fin (12′) of claim 1 further comprising: nine raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′); each of said nine raised vanes (112) formed between first and second longitudinal slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet; wherein first and second vanes (112) of said nine raised vanes (112) are disposed in a first row (120) of vanes (112) that is parallel to said first rib (100), third and fourth vanes (112) of said nine raised vanes (112) are disposed in a second row (122) of vanes (112) that is parallel to said first rib (100), a fifth vane (112) of said nine raised vanes (112) is disposed in a third row (124) of vanes (112) that is parallel to said first rib (100), said second row (122) of vanes (112) is disposed adjacent to and between said first and third rows (120, 124) of vanes (112), sixth and seventh vanes (112) of said nine raised vanes (112) are disposed in a fourth row (126) of vanes (112) that is parallel to said first rib (100), said third row (124) of vanes (112) is disposed adjacent to and between said second and fourth rows (122, 126) of vanes (112), eighth and ninth vanes (112) of said nine raised vanes (112) are disposed in a fifth row (128) of vanes (112) that is parallel to said first rib (100), and said fourth row (126) of vanes (112) is disposed adjacent to and between said third and fifth rows (124, 128) of vanes (112).
 8. The fin (12′) of claim 3 wherein: each of said plurality of raised vanes (112) has a depth dimension (d_(v)) from said first slit (110) to said second slit (110) between 0.5 and 1.5 millimeters.
 9. The fin (12′) of claim 3 wherein. each of said plurality of raised vanes (112) has a height dimension (h_(v)) from said upper surface (103) of said sheet to said top surface of said vane (112) between 0.25 and 0.75 millimeters.
 10. The fin (12′) of claim 1 wherein: each of said ribs (100) has a height dimension (h_(r)) from said upper surface (103) of said sheet to the top surface of said rib (100) between 0.05 and 0.25 millimeters.
 11. The fin (12′) of claim 1 wherein: the longitudinal distance (p_(w)) between the centers of two adjacent apertures (18′) of said first plurality of apertures (18′) in said first longitudinal row of apertures (18′) is between 12 and 18 millimeters.
 12. The fin (12′) of claim 2 wherein: the perpendicular distance (p_(t)) between the center of said first longitudinal row of apertures (18′) and the center of the second longitudinal row of apertures (18′) is between 10 and 15 millimeters.
 13. A fin (12′) for a tube and fin heat exchanger (10) comprising: a generally planar metallic sheet; a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′); and a plurality of raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′); each of said plurality of raised vanes (112) formed between first and second parallel slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet.
 14. The fin (12′) of claim 13 wherein: first and second vanes (112) of said plurality of raised vanes (112) are disposed in a first row (120) of vanes (112) that is parallel to said first longitudinal row of apertures (18′); third and fourth vanes (112) of said plurality of raised vanes (112) are disposed in a second row (122) of vanes (112) that is parallel to said first longitudinal row of apertures (18′); a fifth vane (112) of said plurality of raised vanes (112) is disposed in a third row (124) of vanes (112) that is parallel to said first longitudinal row of apertures (18′); sixth and seventh vanes (112) of said plurality of raised vanes (112) are disposed in a fourth row (126) of vanes (112) that is parallel to said first longitudinal row of apertures (18′); eighth and ninth vanes (112) of said plurality of raised vanes (112) are disposed in a fifth row (128) of vanes (112) that is parallel to said first longitudinal row of apertures (18′); said second row (122) of vanes (112) is disposed adjacent to and between said first and third rows (120, 122) of vanes (112); said third row (124) of vanes (112) is disposed adjacent to and between said second and fourth rows (122, 126) of vanes (112); and said fourth row (126) of vanes (112) is disposed adjacent to and between said third and fifth rows (124, 128) of vanes (112).
 15. The fin (12′) of claim 14 wherein: said first, second, third, fourth, sixth, seventh. eight, and ninth vanes (112) of said plurality of raised vanes (112) each have first and second distal ends (113) connected to said sheet, each of said distal ends (113) being oriented at an angle (α) between 15 and 45 degrees from an imaginary line that is perpendicular to said first longitudinal row of apertures (18′).
 16. A tube and fin heat exchanger (10) comprising: a plurality of fins (12′) arranged in a stack, each of said plurality of fins (12′) characterized by a generally planar metallic sheet, a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′), and first and second longitudinal ribs (100) formed in said sheet, said first longitudinal row of apertures (18′) disposed between said first and second longitudinal ribs (100), said first and second ribs (100) each having a top surface projecting beyond the upper surface (103) defined by said sheet; and a tube (22) received through said stack and in physical contact with each of said plurality of fins (12′).
 17. A tube and fin heat exchanger (10) comprising: a plurality of fins (12′) arranged in a stack, each of said plurality of fins (12′) characterized by a generally planar metallic sheet, a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′), and a plurality of raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′), each of said plurality of raised vanes (112) formed between first and second parallel slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet; and a tube (22) received through said stack and in physical contact with each of said plurality of fins (12′). 